Bursting at the seams: the star-forming main sequence and its scatter at z=3-9 using NIRCam photometry from JADES

TL;DR

Using JWST/NIRCam photometry from JADES and Prospector SED fitting, this work measures the SFMS and its intrinsic scatter from z = 3 to z = 9 on a stellar-mass complete sample, finding sSFR_MS ∝ (1+z)^{μ} with μ ≈ 2.30 for 10 Myr averaging. The SFMS normalization increases with shorter SFR averaging due to bursty SFHs and rising SFHs, while the intrinsic scatter drops from ~0.4–0.5 dex at 10 Myr to ~0.2 dex at 100 Myr, indicating short-term variability dominates. Bursty SFHs are more pronounced at lower masses, and although UV variability at z ≤ 9 aligns with some models, additional mechanisms are required to explain the UV-bright galaxy excess at z > 10. The study also highlights how stellar-mass completeness critically affects SFMS fits and provides constraints on the role of burstiness in cosmic star formation and reionisation.

Abstract

We present a comprehensive study of the star-forming main sequence (SFMS) and its scatter at redshifts $3 \leq z \leq 9$, using NIRCam photometry from the JADES survey in the GOODS-S and GOODS-N fields. Our analysis is based on a sample of galaxies that is stellar mass complete down to $\log \left(M_{\star}/M_{\odot}\right) \approx 8.1$. The redshift evolution of the SFMS at an averaging timescale of 10 Myr follows a relation, quantified by the specific star-formation rates (sSFR$_{10}$), of $\mathrm{sSFR}\propto(1+z)^μ$ with $μ= 2.30^{+0.03}_{-0.01}$, in good agreement with theoretical predictions and the specific mass accretion rate of dark matter halos. We find that the SFMS normalisation varies in a complex way with the SFR averaging timescale, reflecting the combined effects of bursty star formation and rising star formation histories (SFHs). We quantify the scatter of the SFMS, revealing that it decreases with longer SFR averaging timescales, from $σ_{\rm{int}} \approx 0.4-0.5~\mathrm{dex}$ at 10 Myr to $σ_{\rm{int}} \approx 0.2~\mathrm{dex}$ at 100 Myr, indicating that shorter-term fluctuations dominate the scatter, although long-term variations in star formation activity are also present. Our findings suggest that bursty SFHs are more pronounced at lower stellar masses. Furthermore, we explore the implications of our results for the observed over-abundance of UV-bright galaxies at $z > 10$, concluding that additional mechanisms, such as top-heavy initial mass functions, increased star-formation efficiencies, or increased burstiness in star formation are needed to explain these observations. Finally, we emphasize the importance of accurate stellar mass completeness limits when fitting the SFMS, especially for galaxies with bursty SFHs.

Figures (20)

• Figure 1: Representative examples of the products of our SED fitting routine with Prospector. The names and redshifts for each galaxy are indicated in the titles. The names are composed by the coordinates of each galaxy rounded to the fifth decimal. The observed photometry is shown as open squares (HST) and circles (JWST NIRCam), while the best-fit photometry is shown as orange diamonds, and the best-fit spectra is shown in grey. We also show the UV continuum slope Calzetti1994, obtained by fitting a line to the best-fit spectra in logarithmic space at $\lambda_{\rm{rest-frame}}=1250-2500$Å (purple hatched region). The vertical dotted lines show the expected wavelength of selected emission lines. Finally, the insets show the star formation history and an RGB image of the selected galaxy. These examples illustrate the power of Prospector to reproduce the observed SED shapes of galaxies even in the absence of strong emission lines. Top panel: galaxy at $z\sim 3.5$ with no evidence of strong emission lines and a declining SFH. Bottom panel: galaxy at $z\sim 8.5$ with strong evidence of [O iii]$_{\lambda5007}$ and a recent burst in star formation.
• Figure 2: Comparison of Prospector-inferred H$\alpha$ (circles) and [O iii]$_{\lambda5007}$ (crosses) emission line fluxes to ones measured from JADES using NIRSpec. The bottom panel shows the difference between the inferred and measured values including their respective errors, given by: $\text{E}\equiv(\text{F}_{\rm{Prospector}}-\text{F}_{\rm{NIRSpec}})/(\sqrt{\sigma_{\rm{Prospector}}^{2}+\sigma_{\rm{NIRSpec}}^{2}})$. The dotted lines denote the $3\sigma$ range, measurements outside of these limits are considered outliers. We find an outlier fraction of 3.2% for H$\alpha$ and 21.5% for [O iii]. Finally, the JADES measurements might be underestimated in bright galaxies due to slit losses. Overall, we find good agreement between our fitted values and the observed ones, as shown by the best fit to the data (shaded area).
• Figure 3: Stellar mass completeness of the sample, divided into three fields depending on exposure time (T$_{\mathrm{exp}}$). The larger grey circles show the stellar mass of each galaxy as a function of redshift, while the smaller coloured circles show the limiting mass for the faintest 20%. The solid curves denote the 100% completeness at each redshift and for each depth, as indicated in the legend, following the prescription of Pozzetti2010. We find our sample is stellar mass complete down to log(M$_{\star}$/[M$_{\odot}$]) $\approx 8.1$ on average, with small variations depending on the depth of the field. The dotted vertical lines mark the redshift bins used in the mass completeness estimation. The blue hatched area shows the region used to fit the star-forming main sequence (i.e. $9.0\leq$log(M$_{\star}$/M$_{\odot}$)$\leq10.3$ and $3\leq z\leq9$.)
• Figure 4: Stellar mass range and redshift evolution used to fit the star forming main sequence. Left panel: specific star formation rate (sSFR$_{10}$) using the star formation averaged over the past 10 Myr (SFR$_{10}$), as a function of stellar mass for all galaxies in our sample with log(M$_{\star}$/M$_{\odot}$)$\geq 7.0$, colour-coded by redshift. The white circles with blue edges show the medians per stellar mass bins (0.2 width in logarithmic space). The vertical lines show different stellar mass completeness limits, delimiting three regions. The upper limit of the region labelled "incomplete" is given by the mean stellar mass completeness estimated in Figure \ref{['fig:mass_completeness']} (log(M$_{\star}$/M$_{\odot}$)$\approx 8.1$). We note that the median sSFRs increase steadily below this limit. This trend is consistent with an increasing prevalence of rising SFHs at lower stellar masses, but it is also expected in biased samples that are incomplete in stellar mass. Interestingly, this behaviour continues (although less steeply) up to log(M$_{\star}$/M$_{\odot}$)$=9.0$, which we dub partially complete ("PC"). The shaded area shows galaxies with $9.0\leq$log(M$_{\star}$/M$_{\odot}$)$\leq10.3$ (also shown as a blue hatched area in Figure \ref{['fig:mass_completeness']}), we note that the sSFR flattens in this mass range, and thus we define this region as being complete ("C"), and use all galaxies in it to fit the star forming main sequence (i.e., without imposing a SFR range). The upper limit of the complete region is motivated by the turnover of the SFMS at log(M$_{\star}$/M$_{\odot}$)$=10.3$. Finally, the hatched horizontal area shows the upper limit allowed given the timescale used. Right panel: median sSFRs for galaxies with $9.0\leq$log(M$_{\star}$/M$_{\odot}$)$\leq9.5$ as a function of redshift, the mass bin was selected because it is populated at all redshift bins. To increase numbers, we merge the two highest redshift bins ($7\leq z\leq 9$). We fit a line to all medians to obtain the redshift evolution, $\mu_{10}$, which we use as input when fitting the SFMS (see Equation \ref{['eq:MS']}). The redshift evolution for all the remaining timescales analysed in this work are shown as dotted lines.
• Figure 5: Specific star formation rate, sSFR$_{10}$, as a function of stellar mass, colour-coded by their ionising photon production efficiency ($\xi_{\rm{ion},0}$), and divided by redshift bins. The thick black dashed lines show the best fit to the star-forming main sequence given in Equation \ref{['eq:MS']}, for galaxies with stellar masses in the range $9.0 \leq \log(\text{M}_{\star}/\text{M}_{\odot}) \leq 10.3$ (indicated by the grey shaded region bounded by vertical dashed lines), and are extrapolated to all stellar masses in our sample. To improve the robustness of the fit at high redshift, the two highest redshift bins were grouped together when deriving the best-fit relations, though they are shown separately in the figure for consistency with the redshift binning used throughout the paper. The blue filled lines show piece-wise fits to the same equation in mass bins of stellar masses log(M$_{\star}$/M$_{\odot}$)=8.0-10.0 in steps of 0.5 in logarithmic space. The grey hatched region shows the upper limit in sSFR, given by $\frac{1}{\text{t}_{\rm{average}}}$. For reference, we include the stellar mass completeness limits shown in Figure \ref{['fig:mass_completeness']}: from left to right, the vertical dotted lines show the ultra-deep, deep, and medium exposure times. We note that if we fit the SFMS using these limits instead (log(M$_{\star}$/M$_{\odot}$)$\gtrapprox 8.1$), the slope of the fit becomes negative, as indicated by the discrepancy between the piece-wise fit and the total fit in that stellar mass regime.